The two principal attributes of the ideal internal combustion engine are high thermal efficiency and low emissions. However, in both two-and four-cycle engines of conventional design, the presence of unstratified exhaust gas in the combustion chamber interferes with the burning of the fuel air mixture supplied to the engine and combustion is relatively poor. This problem is particularly acute at idle and light load, and current attempts to reduce the emission of NO.sub.x, CO and HC tend to reduce the efficiency to even lower levels than unmodified engines.
It is accordingly an object of this invention to provide an arrangement whereby fuel is burned efficiently in stratified relation to exhaust gas, and stratified excess air is provided to complete the burning process early in the combustion cycle. It will be shown that whereas two-cycle engines in conventional form have been particularly handicapped by excessive emissions, particularly of HC, they can, with proper stratification, avoid this problem, and because of the large quantity of recycled exhaust gas inherent in their operation, can also be low in NO.sub.x emissions. It is accordingly a further object of this invention to provide a two-cycle engine which retains this feature of two-cycle engines and also obtains a fast, efficient and clean combustion under all operating conditions.